What is the difference between interxylary and intraxylary phloem?
The phloem is a component of the vascular tissue system in plants responsible for the conduction of soluble organic materials. Its counterpart, the xylem, conducts water and mineral nutrients from the roots to the shoot and leaves. Both elements of the vascular bundle originate from the procambium. In dicotyledons and gymnosperms where the stems grow in thickness, part of the procambium develops into the cambium.
Phloem is external to the xylem in the vascular bundle except in certain dicotyledonous plants such as the Cucurbitaceae family where the phloem may also exist internal to the xylem. In such cases, the internal phloem growing inside the xylem was previously referred to as intraxylary phloem by early researchers. Both the internal and the external phloems are similar in structure and composition and so the term intraxylary phloem is no longer used.
The interxylary phloem on the other hand is a different type of phloem which exists in some dicotyledonus families such as Combretaceae and Acanthaceae. They are embedded in the secondary xylem, which is formed during secondary growth from the vascular cambium. They are made up of sieve tubes, companion cells and other adjacent cells embedded within the secondary xylem and they are histologically distinct from the intraxylary phloem.